15... g8h8 The rook goes back to the now-open file. It's worth remembering that only this rook has moved, so castling queenside is still possible, although not advisable. I must admit that I once forgot that the rook had already moved and castled anyway, to my opponent's surprise - probably many have a similar story to tell. 16. b2b4  16... f6h5  17. a4a5  17... b6d8 